1. Field of the Invention
The present invention relates to a flat display panel constructed as a hermetically sealed case, such as a plasma display panel, to a flat display device having the flat display panel, and to a method for forming the flat display panel.
2. Description of the Background Art
FIG. 15 shows the structure of an AC-type plasma display device as an example of a flat display device.
In FIG. 15, the AC-type plasma display device 201 has a plasma display panel 111, a plasma display panel driving circuit 202, address electrodes 204, X electrodes 205, and Y electrodes 206. The address electrodes 204, X electrodes 205 and Y electrodes 206 are all plural in number. The electrodes are arranged so that the address electrodes 204 perpendicularly intersect the X and Y electrodes 205 and 206 in a plurality of discharge cells 203 arranged to form a grid in the image display portion 111a of the plasma display panel 111.
The address electrodes 204, X electrodes 205 and Y electrodes 206 are all connected to the plasma display panel driving circuit 202 and supplied with driving voltage from the plasma display panel driving circuit 202.
To obtain a desired image in this AC-type plasma display device 201, first, the plasma display panel driving circuit 202 performs addressing operation. More specifically, for the addressing operation, write voltage is applied between the address electrodes 204 and the Y electrodes 206, for example. This causes write discharge between these electrodes to set discharge cells 203 involved in the display operation. As is well known, this operation is write operation in which wall charge is stored in the dielectric in the plasma display panel of the AC-type plasma display device.
Subsequently, the plasma display panel driving circuit 202 performs discharge sustain operation (display operation). More specifically, for the discharge sustain operation, the discharge cells 203 set by the addressing operation are caused to discharge to present a display. For this purpose, sustain voltage is alternately applied between the X electrodes 205 and the Y electrodes 206. This discharge sustain operation causes discharge between the X electrodes 205 and the Y electrodes 206 in the discharge cells 203, displaying an image in the image display portion 111a. 
When given discharge sustain operation ends, the plasma display panel driving circuit 202 performs erasing operation to change the image displayed in the image display portion 111a (operation for eliminating the wall charge). More specifically, erase voltage is applied between the X electrodes 205 and the Y electrodes 206 to eliminate the wall charge.
FIG. 16 shows the structure of a conventional flat display panel 11, e.g. the plasma display panel 111. FIG. 16 is a top view of the flat display panel 11. FIG. 17 is the sectional view taken along the line Bxe2x80x94B of FIG. 16. FIG. 18 is a sectional view of the flat display panel 11 in a stage where it has not yet been heated and processed into the condition shown in FIG. 17.
The flat display panel 11 has two substrates 1A and 1B made of glass etc. and a sealing layer 12 for bonding the substrates 1A and 1B together. In the case of a plasma display panel, for example, the substrates 1A and 1B are a display surface glass substrate and a back glass substrate opposing each other. The sealing layer 12 is arranged near and along the peripheries of the substrates 1A and 1B to keep the image display portion 11a in a hermetic state.
FIG. 16 shows the substrate 1B with a broken line so that the sealing layer 12 can be seen. FIG. 17 shows barrier ribs 13 sectioning individual discharge cells in the plasma display panel as an example of components in the image display portion 11a. 
A glass paste is generally used as the material of the sealing layer 12; for example, a thermally soluble material, in which powder of a low-melting-point glass (frit glass) such as PbOxe2x80x94B2O3xe2x80x94SiO2 type glass or PbOxe2x80x94B2O3xe2x80x94ZnO type glass, is mixed into a solvent together with a binder of nitrocellulose or acrylic resin, a filler of ceramics powder for adjusting the thermal expansion coefficient to those of the substrates 1A and 1B, and so forth. In this specification, xe2x80x9cfrit glassxe2x80x9d stands for glass materials having lower melting points than ordinary glass; e.g. a glass material having a melting point around 400xc2x0 C. In a broader sense, it stands for glass materials which melt at lower temperatures than the substrates 1A and 1B.
A common sealing procedure is now described referring to the plasma display panel as an example. (1) First, the sealing layer 12 is formed on the display surface glass substrate or on the back glass substrate (on the substrate 1B in the example of FIG. 18) where internal components like the barrier ribs 13 have been previously formed. (2) Pre-firing is applied to cause desorption of the binder component in the sealing layer 12. (3) Next, positioning is achieved with the display surface glass substrate and the back glass substrate facing each other. (4) The two glass substrates are fixed with a jig like a clip and appropriate pressure is applied to the sealing layer 12. (5) The entire panel is heated. (6) The panel is cooled and the jig or clip is removed. (7) When the sealing is completed, the entire panel is evacuated, while being heated, through an exhaust tube previously attached to the panel, so as to remove impurity gas adsorbed in the panel. (8) A gas for discharge is entrapped (a mixture gas containing Ne, Xe, etc.) when the panel has reached a given temperature. (9) The exhaust tube is sealed.
Generally, the sealing layer 12 is required to satisfy the following conditions: (a) to have such fluidity that it will easily deform and fuse upon application of external pressure at the sealing temperature, (b) to have such rigidity that it will not deform by atmospheric pressure at the evacuating temperature, (c) to have thermal expansion coefficient at the same level as those of the display surface glass substrate and the back glass substrate so that the substrates will not crack during the sealing process and after the cooling process.
To satisfy the conditions above, the sealing layer 12 has generally been formed by using an amorphous glass paste which contains amorphous frit glass or using a crystallized glass paste which contains crystallized frit glass. The amorphous glass paste has superior fluidity and is not very susceptible to temperature condition. The crystallized glass paste, on the other hand, is poor in fluidity, but provides excellent thermal resisting stability after it is sealed.
Whichever glass paste is used, however, the sealing layer cannot sufficiently alleviate and absorb strain stress between the two substrates which is caused by the internal stress of the two substrates while they are being sealed and after they have been cooled. It is therefore difficult to obtain a large quantity of flat display panels with sufficiently ensured hermetic seal; for example, the hermetic seal of the flat display panel may be broken if the flat display device undergoes external force, such as vibrations and impacts, during its assembly or transportation after cooling, which results in lower yield.
Furthermore, the amorphous glass paste has lower softening point than the crystallized glass paste. Therefore, during the exhausting process following the sealing process, it requires that the temperature be set lower than when the crystallized glass paste is used, which may result in insufficient removal of the impurity gas. The temperature must be set lower because the bonded sealing part will otherwise be re-softened with heat during the exhaust and then the dynamic bonding strength will be reduced or the hermetic seal will be broken to cause leakage of the discharge gas.
On the other hand, the crystallized glass paste, having higher softening point than the amorphous glass paste, has to be heated for a longer time period than the amorphous glass paste to cause crystallization. Moreover, when the temperature distribution largely varies during the heating process, parts where the temperature increases slower will melt while being affected by stress (strain) generated in parts which have hardened earlier, because of the characteristic of the material itself that the melting and the hardening by crystallization are completed where the temperature has increased earlier. It is therefore difficult to uniformly bond the entire panel together.
Furthermore, the bonding part is required to maximize the area of the image display portion while minimizing the width of the sealing layer (the area it occupies on the substrates). When the width of the sealing layer is large, the area of the image display portion will be reduced, or the sealing layer may come in contact with the barrier ribs etc. in the image display portion, and then the path of gas cannot be ensured during the exhaust process. Therefore it is also necessary that the sealing layer provide as uniform width as possible after it is heated.
However, when the crystallized glass paste having poor fluidity is used, it is very difficult to set conditions for the application of external force, so that the sealing layer may provide uneven thickness (uneven flattening) after it has been heated.
Moreover, it has not been clearly known where the jig should be located on the two glass substrates in the step (4) in the above-described sealing process, i.e. in the process step where appropriate pressure is applied to the sealing layer 12 with the two glass substrates fixed with a jig.
FIGS. 19 and 20 show examples of the relation between the position where the pressure is applied and the position where the sealing layer 12 forms; the two substrates 1A and 1B are put into a heating furnace while being pressed by external pressure PS.
In FIG. 19, the distance xe2x80x9caxe2x80x9d from the end of the substrate 1A to the position where the pressure PS is applied and the distance xe2x80x9cbxe2x80x9d from the end of the substrate 1A to the center of the formation width of the sealing layer 12 are set in the ratio of a=b. When the distances are set as a=b, unnecessarily large pressure is applied to the sealing layer 12 and the edges of the glass substrates 1A and 1B will be deformed after they are heated. As a result, the sealing layer 12 is formed in a larger width and hence at a smaller distance from the barrier ribs etc. in the image display portion, in which case it is difficult to ensure the path of gas during the exhaust process.
In FIG. 20, the distance xe2x80x9caxe2x80x9d from the end of the substrate 1A to the position of application of the pressure PS and the distance xe2x80x9cbxe2x80x9d from the end of the substrate 1A to the center of the formation width of the sealing layer 12 are set in the ratio of a greater than  greater than b. When the distances are set as a greater than  greater than b, insufficient pressure is applied to the sealing layer 12 and then conditions for bonding the substrates 1A and 1B together cannot be satisfied. If the worst happens, a gap GP may be formed to cause leakage of the discharge gas.
A first aspect of the present invention is directed to a flat display panel comprising: first and second substrates; and a plurality of sealing layers which adjoin each other, wherein the first and second substrates are sealed together with the plurality of sealing layers and the plurality of sealing layers have different thermal expansion coefficients from each other.
Preferably, according to a second aspect, in the flat display panel, the plurality of sealing layers form a stacked structure between the first and second substrates.
Preferably, according to a third aspect, in the flat display panel, the plurality of sealing layers are arranged side by side between the first and second substrates.
Preferably, according to a fourth aspect, in the flat display panel, the plurality of sealing layers are arranged in a line between the first and second substrates.
Preferably, according to a fifth aspect, in the flat display panel, the plurality of sealing layers comprise a first sealing layer comprising a crystallized glass paste and a second sealing layer comprising an amorphous glass paste.
Preferably, according to a sixth aspect, in the flat display panel, the plurality of sealing layers comprise a first sealing layer having a first thermal expansion coefficient, a second sealing layer having a second thermal expansion coefficient which is lower than the first thermal expansion coefficient, and a third sealing layer provided between the first sealing layer and the second sealing layer and having a third thermal expansion coefficient which is lower than the first thermal expansion coefficient and higher than the second thermal expansion coefficient.
Preferably, according to a seventh aspect, in the flat display panel, the plurality of sealing layers comprise a first sealing layer having a first softening point and a second sealing layer having a second softening point which is lower than the first softening point.
Preferably, according to an eighth aspect, in the flat display panel, the plurality of sealing layers further comprise a third sealing layer provided between the first sealing layer and the second sealing layer, the third sealing layer having a third softening point which is lower than the first softening point and higher than the second softening point.
Preferably, according to a ninth aspect, in the flat display panel, irregularities exist at an interface between the plurality of sealing layers.
A tenth aspect of the present invention is directed to a flat display device comprising: the flat display panel of the first aspect and a flat display panel driving circuit for controlling driving of the flat display panel.
An eleventh aspect of the present invention is directed to a flat display panel manufacturing method comprising a process of providing a sealing layer between first and second substrates and sealing the first and second substrates together by externally applying a pressing force to the first and second substrates to press the first and second substrates together, wherein the pressing force applied in the sealing process is positioned near the sealing layer and inside the position of the sealing layer.
According to the first aspect of the present invention, the plurality of sealing layers adjoining each other have different thermal expansion coefficients from each other. The thermal expansion coefficients of the plurality of sealing layers are set approximately equal respectively to those of the first and second substrates, to the extent that the seal between the first and second substrates will not be broken, and the plurality of sealing layers are appropriately arranged between the first and second substrates. Thus the plurality of sealing layers can gradually alleviate and absorb the strain stress between the two substrates which is caused by the internal stress of the first substrate and the internal stress of the second substrate. This provides a flat display panel with a stabler hermetic seal as compared with a flat display panel having only one sealing layer. The flat display panel can thus be kept hermetically sealed even if the flat display device is subjected to external forces, such as vibrations and shocks, during its assembly process or transportation after cooling, for example.
According to the second aspect, the plurality of sealing layers form a stacked structure between the first and second substrates, which can gradually alleviate and absorb the strain stress between the two substrates which is caused especially by internal stress of the first and second substrates directed in their thickness direction.
According to the third aspect, the plurality of sealing layers are arranged side by side between the first and second substrates, which can gradually alleviate and absorb the strain stress between the two substrates which, especially, is caused by internal stress traversing the sealing layers in the surfaces of the first and second substrates.
According to the fourth aspect, the plurality of sealing layers are arranged in a line between the first and second substrates, which can gradually alleviate and absorb the strain stress between the two substrates which, especially, is caused by internal stress directed in the direction of the line in which the sealing layers are disposed on the surfaces of the first and second substrates.
According to the fifth aspect, the plurality of sealing layers include a first sealing layer containing a crystallized glass paste and a second sealing layer containing an amorphous glass paste. It is therefore possible to make full use of advantages of the two pastes while canceling out their disadvantages.
According to the sixth aspect, a third sealing layer having a third thermal expansion coefficient lower than the first thermal expansion coefficient and higher than the second thermal expansion coefficient is provided between the first sealing layer and the second sealing layer. It is therefore possible to more gradually alleviate and absorb the strain stress between the two substrates which is caused by the internal stress of the first substrate and the internal stress of the second substrate. This provides a flat display panel with a stabler hermetic seal.
According to the seventh aspect, the plurality of sealing layers include a first sealing layer having a first softening point and a second sealing layer having a second softening point lower than the first softening point. Therefore, in the cooling process performed during formation of the plurality of sealing layers, the first sealing layer solidifies first and the second sealing layer solidifies later. Accordingly, in the case of the flat display panel of the second aspect, the second sealing layer can alleviate and absorb strain stress between the substrates which the first sealing layer cannot completely alleviate and absorb. Further, in the case of the flat display panel of the third aspect, the first sealing layer will not be re-softened while being heated in the exhausting process, even though the second sealing layer may be re-softened, which enables stable heating exhaust.
According to the eighth aspect, a third sealing layer having a third softening point lower than the first softening point and higher than the second softening point is provided between the first sealing layer and the second sealing layer. Accordingly, in the case of the flat display panel of the second aspect, the third sealing layer can alleviate and absorb strain stress between the two substrates which the first sealing layer cannot completely alleviate and absorb, and the second sealing layer can alleviate and absorb strain stress between the two substrate which the third sealing layer cannot completely alleviate and absorb. Further, in the case of the flat display panel of the third aspect, the first and third sealing layers will not be re-softened while heated in the exhausting process even though the second sealing layer may be re-softened, which enables stable heating exhaust.
According to the ninth aspect, an interface between the plurality of sealing layers have irregularities and therefore stresses of the sealing layers cancel each other at the interface, which suppresses separation between the sealing layers. This provides a flat display panel with a stabler hermetic seal.
The tenth aspect provides a flat display device having the effects of the flat display panel of the first aspect.
According to the eleventh aspect, the position of application of the pressing force applied during the sealing process is set near the sealing layer and inside the position of the sealing layer. This prevents application of excessive pressure to the sealing layer and thereby prevents the width of the sealing layer from being unnecessarily enlarged. It is therefore easy to ensure the passage of the gas exhausted from the cavity between the first and second substrates. This also suppresses application of insufficient pressure to the sealing layer. Therefore gaps are less apt to form between the first and second substrates. Furthermore, the sealing layer can be formed in almost uniform width along the entire periphery, which prevents nonuniform occurrence of the internal stresses. Moreover, the sealing layer occupies smaller area while maximizing the display area, and also provides uniform sealing width after sealed, as is required for bonding. Deformation of the substrate edges and leakage of the discharge gas are least likely to occur when the ratio between the distance from the substrate edge to the pressing force application position and the distance from the substrate edge to the center of the sealing layer formation width is approximately in the range of 2:1 to 10:1.
The present invention has been made to solve the problems explained earlier, and objects of the present invention are to provide a flat display panel and a flat display device in which the sealing layer can sufficiently alleviate and absorb the strain stress between two substrates which is caused by the internal stresses of the substrates during sealing process and after cooling process, and which can cancel out disadvantages of an amorphous glass paste and a crystallized glass paste, and to provide a flat display panel manufacturing method which prevents application of excessive or insufficient force to the sealing layer.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.